closes#2896
- move default options / custom code into model functions
- move most of the filtering logic into base/utils.filtering (to be relocated)
- move the remainder of findPage back into base/index.js and remove from posts/users&tags
- move pagination-specific logic to a separate 'plugin' file
- pagination provides new fetchPage function, similar to fetchAll but handling pagination
- findPage model method uses fetchPage
- plugin is fully unit-tested and documented
